chore(all): return concrete types, accept interfaces

- Remove exported interfaces unused locally
- Define interfaces to accept arguments
- Return concrete types, not interfaces

internal/cli/cli.go

@@ -1,16 +1,6 @@
 // Package cli defines an interface CLI to run command line operations.
 package cli
 
-var _ CLIer = (*CLI)(nil)
-
-type CLIer interface {
-	ClientKeyFormatter
-	HealthChecker
-	OpenvpnConfigMaker
-	Updater
-	ServersFormatter
-}
-
 type CLI struct {
 	repoServersPath string
 }